1. Field of the Invention
The present invention relates to a method and apparatus for coating a substrate, particularly a glass substrate. More particularly, the invention relates to a continuous chemical vapor deposition (CVD) process for producing coated glass for vehicles or architectural windows, and the apparatus for producing such coated glass.
2. Summary of Related Art
Chemical vapor deposition processes are well known in the art of coating substrates. The substrate is coated by directing a flow of a gaseous reactant mixture toward the surface of the substrate to cause the reactant mixture to deposit the desired coating on the substrate surface. The physical form of the coating reactants employed in such processes may be liquid, vapor, liquids or solids dispersed in gas mixtures, aerosols, or vaporized or vaporous coating reactants dispersed in gas mixtures.
One of the problems associated with CVD processes relates to the production of a coating of uniform thickness along the width of the substrate. In certain instances, such as the application of a reflective metal oxide coating to a sheet of glass, the requirement that the coating have a uniform thickness is especially important, since small variations in the coating thickness can result in the glass sheet having non-uniform color. For example, variations in thickness of the metal oxide coating of one millionth of an inch can produce an unaesthetic appearance in the glass.
Generally, in order to produce a coating having a constant thickness, it is necessary that the gaseous reactant mixture be uniformly applied to the entire sheet of glass. Various means have been proposed for applying the gaseous reactant mixture in a uniform manner to a sheet of glass. For example, U.S. Pat. No. 4,504,526 discloses an apparatus which comprises a plurality of serially arranged, individual passageways interconnecting an inlet and an outlet positioned adjacent the glass sheet. Each of the individual passageways includes a portion having a cross sectional area different from the cross sectional area of adjacent passageways, causing the velocity of the fluid flowing through the passageways to vary.
In U.S. Pat. No. 4,535,000, Gordon describes a chemical vapor deposition process for applying a thin metal nitride film on a glass substrate. Gordon suggests the use of multiple coaters as a means of obtaining a uniform coating, since the non-uniformities of one coater will not usually match those of the other coaters, and there will tend to be some cancellation of thickness errors from the different coaters.
As taught by Sopko et al. in U.S. Pat. No. 3,580,679, a gaseous mixture is distributed along the substrate by a vapor manifold comprised of a plurality of vapor channels separated from one another at their exit ends, but which meet in a common channel at their entrance. To enhance the uniformity of vapor deposition, it is suggested that each channel include at least two opposing curves so that the vapors passing through the channel change direction at least twice. In addition, Sopko notes that baffles may be positioned within the channels to further interrupt the vapor flow and distribute the vapor along the length of the vapor manifold.
Despite the various means previously proposed, some examples of which are noted above, problems still arise with regard to the application of a coating of uniform thickness on a substrate by a chemical vapor deposition process. The causes of such non-uniformity are uncertain, but may include problems of exhaust balancing, temperature gradients in the substrate to be coated, dimensional changes in the coater upon heating, changes in the entrance region Reynolds number with changes in total flow, and the build up of deposits in the coater. Whatever the cause, it would be advantageous to provide a method and apparatus for substantially eliminating any lack of uniformity in the thickness of a coating applied to a substrate in a chemical vapor deposition process.